Exhaust gas after-treatment unit with countercurrent housing and corresponding process for exhaust gas after-treatment

ABSTRACT

An exhaust gas after-treatment unit, in particular for use close to an internal combustion engine of an automobile, includes a housing which contains at least one catalytic converter surrounded by at least one substantially free flow-through return flow region. The catalytic converter has first and second end surfaces and hollow spaces through which a fluid can flow in an inflow direction. The first end surface is connected to at least one gas feed line and at least one gas removal line is substantially gas-tightly connected to the return flow region. At least one flow deflector effects a deflection of the fluid from the catalytic converter into the return flow region of the housing. The exhaust gas after-treatment unit has a compact construction, improved start-up properties and lower thermal alternating stresses as compared with conventional exhaust gas after-treatment units.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuing application, under 35 U.S.C. § 120, of copending International Application No. PCT/EP2004/006204, filed Jun. 9, 2004, which designated the United States; this application also claims the priority, under 35 U.S.C. § 119, of German Patent Application 103 29 000.1, filed Jun. 27, 2003; the prior applications are herewith incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The invention relates to an exhaust gas after-treatment unit with a countercurrent housing. The invention also relates to a corresponding process for exhaust gas after-treatment.

Legal limits which must not be exceeded by the pollutant load of the exhaust gas of automobiles have been adopted in numerous countries throughout the world, due to the constantly increasing extent of automobile traffic. Those limits are lowered on a regular basis, so that there must be an increased expenditure for conversion of pollutants in exhaust gas to comply with the limits. It has become accepted in that context to subject the exhaust gas to a catalytic conversion, in which noxious contents of the exhaust gas are converted into harmless contents. Such a catalytic conversion requires the largest possible reaction surface, but the component used therefor should not be so large that it exceeds the space conventionally available in an automobile. Honeycomb bodies used as a catalyst carrier body offer a solution to that problem. Honeycomb bodies have hollow spaces, for example channels, over or through which the exhaust gas can flow. A large reaction surface for the catalytic conversion can be provided by the construction of walls which separate the hollow spaces and can be provided with a layer, e.g. a washcoat layer, which includes a catalyst, for example a noble metal catalyst.

Such honeycomb bodies or catalytic converters can be constructed, for example, from ceramic materials, from metallic layers or as an extruded component. A distinction is made above all between two typical structural forms for metallic honeycomb bodies. An early structural form, for which German Published, Non-Prosecuted Patent Application 29 02 779 A1, corresponding to U.S. Pat. No. 4,273,681, shows typical examples, is the spiral structural form, in which substantially a smooth and a corrugated sheet metal layer are laid on one another and are wound up spirally. In another structural form the honeycomb body is constructed from a plurality of alternately disposed smooth and corrugated or differently corrugated sheet metal layers. The sheet metal layers initially form one or more stacks, which are intertwined with one another. In that construction, the ends of all of the sheet metal layers come to lie on the outside and can be connected to a housing or casing tube, as a result of which numerous connections are made which increase the stability of the honeycomb body. Typical examples of those structural forms are described in European Patent 0 245 737 B1, corresponding to U.S. Pat. Nos. 4,946,822; 4,923,109; 4,803,189; and 4,832,998, or International Publication No. WO 90/03220, corresponding to U.S. Pat. Nos. 5,139,844; 5,135,794; and 5,105,539. It has also been known for a long time to equip the sheet metal layers with additional structures in order to influence the flow and/or to achieve a transverse mixing between the individual flow channels. Typical examples of such structures are in International Publication No. WO 91/01178, corresponding to U.S. Pat. No. 5,403,559, International Publication No. WO 91/01807, corresponding to U.S. Pat. Nos. 5,130,208 and 5,045,403, and International Publication No. WO 90/08249, corresponding to U.S. Pat. No. 5,157,010. Finally, there are also honeycomb bodies in a conical structural form, optionally also having further additional structures to influence the flow. Such a honeycomb body is described, for example, in International Publication No. WO 97/49905, corresponding to U.S. Pat. No. 6,190,784. It is moreover also known to leave open a recess for a sensor in a honeycomb body, in particular for accommodating a lambda probe. An example thereof is described in German Utility Model 88 16 154 U1. Honeycomb bodies which render possible a flow of a fluid in the radial direction from the inside outwards are furthermore known. An example thereof which is described in International Publication No. WO 96/09893, corresponding to U.S. Pat. Nos. 5,902,558 and 5,795,658, is formed from discs which lie adjacent one another and have a macrostructure that forms channels which run in an arc shape outwards from a central channel. A further possibility for the construction of honeycomb bodies which have a through-flow radially from the inside outwards is described in International Publication No. WO 98/57050, corresponding to U.S. Pat. No. 6,277,784.

In order to achieve the highest possible rate of conversion and a rapid start-up of the catalytic conversion, it is advantageous to charge the honeycomb body with exhaust gas which is as hot as possible, since upon cold starting in this way it relatively rapidly reaches its start-up temperature from which the catalytic conversion proceeds. This can be achieved by installing the catalytic converter as close to the engine as possible. However, the spaces available for installing a catalytic converter precisely in the region close to the engine are often only very limited. On the other hand, installation close to the engine causes high thermal stress on the catalytic converter because of the thermal gradients formed and the highly pulsatile gas stream.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide an exhaust gas after-treatment unit with a countercurrent housing and a corresponding process for exhaust gas after-treatment, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and processes of this general type and in which the exhaust gas after-treatment can be carried out in a compact manner and rapid start-up properties are ensured, with a simultaneous long life of the exhaust gas after-treatment unit.

With the foregoing and other objects in view there is provided, in accordance with the invention, an exhaust gas after-treatment unit, in particular to be used close to an internal combustion engine of an automobile. The exhaust gas after-treatment unit comprises a housing and at least one catalytic converter disposed in the housing and defining at least one substantially free flow-through return flow region disposed within the housing and surrounding the at least one catalytic converter. The at least one catalytic converter has a first end surface, a second end surface and hollow spaces through which a fluid can flow in an inflow direction. At least one gas feed line is connected to the first end surface of the at least one catalytic converter. At least one gas removal line is substantially gas-tightly connected to the at least one return flow region. At least one flow deflector effects a deflection of the fluid from the at least one catalytic converter into the substantially free flow-through return flow region of the housing.

A substantially free through-flow return flow region is understood herein as meaning in particular that the return flow region is not constructed as a honeycomb structure, i.e. is substantially not divided into through-flow channels or hollow spaces. In particular, it is possible according to the invention for there to be a completely free through-flow return flow region, where appropriate with the exception of fixing devices for fixing the catalytic converter, which is, for example, a honeycomb structure in a casing tube, in the housing. In the case of an internal cylindrical catalytic converter in a cylindrical housing, the return flow region therein is constructed as an annular cylindrical gap between the casing tube of the catalytic converter and the inner wall of the housing. The exhaust gas after-treatment unit according to the invention has the advantage that due to the deflection of the direction of flow, for example, pocket holes in the vicinity of the engine can be used to accommodate the exhaust gas after-treatment unit. It is not possible to use such pocket holes in the case of catalytic converters having a conventional construction—i.e. without deflection of the direction of flow. Since the catalytic conversion as a rule proceeds exothermically, after starting or start-up of the catalytic conversion, the exhaust gas is heated up. This causes severe thermal gradients over the catalytic converter in the case of conventional catalytic converters. Since in an exhaust gas after-treatment unit according to the invention the converted stream of exhaust gas is deflected in its direction of flow, is inverted in a through-flow catalytic converter in the axial direction, and flows back in the return flow region of the housing, but this housing also contains the catalytic converter, the catalytic converter is heated up uniformly, so that thermal gradients are avoided and the service life of the catalytic converter is increased in this way. Heating up of the catalytic converter with the aid of the hot exhaust gas furthermore leads to a faster start-up of the catalytic conversion in the catalytic converter in the cold start phase and thus to significantly accelerated start-up properties as compared with conventional exhaust gas after-treatment units without a countercurrent housing.

In accordance with another feature of the invention, the gas feed line and the gas removal line are provided in the region of the first end surface of the catalytic converter.

The placement of the gas feed line and the gas removal line on only one side of the housing and of the catalytic converter allows a space-saving construction of the exhaust gas after-treatment unit according to the invention. In particular, the gas feed and removal lines are not constructed in parallel, in particular not coaxially. In a catalytic converter through which the exhaust gas flows substantially radially, a deflection of the exhaust gas occurs upon emergence from the catalytic converter, while in an axial through-flow catalytic converter, the deflection of the gas stream represents an inversion of the gas stream, that is to say a deflection of substantially 180° (degrees).

In accordance with a further feature of the invention, the housing is constructed as a manifold or elbow. A further advantageous construction of the exhaust gas after-treatment unit is directed at the construction of the housing as a collector. Both in a construction of the housing as a manifold and in its construction as a collector, it is possible to employ the exhaust gas after-treatment unit as close to the engine as possible.

In accordance with an added feature of the invention, the gas removal line and/or the gas feed line is connected to a turbocharger.

A turbocharger serves to boost the engine, i.e. it is used to increase the performance of an internal combustion engine, and is used in particular in connection with diesel engines. During boosting the air required for the engine combustion process is compressed by a power engine so that a larger mass of air enters the cylinder or combustion chamber per work cycle of the internal combustion engine. For this purpose, the compressor is driven, for example, by a turbocharger which utilizes the exhaust gas energy. The coupling with the engine in this context is not mechanical, but proceeds purely thermally, with the principle of dynamic boosting chiefly being used in automobile construction. The configuration of the exhaust gas after-treatment unit upstream of such a turbocharger ensures that the operating temperature of the catalytic converter contained therein is reached very rapidly, since a removal of heat from the exhaust gas due to contact with components of the turbocharger is avoided in this manner.

The respective configuration of the turbocharger directly connected with or directly upstream of the feed line is, however, particularly preferred. In this construction it is particularly advantageous to provide the feed line with a cone, which conducts the exhaust gas directly to the first end surface of the honeycomb body. This cone advantageously has an opening angle of at least 20°, in particular of at least 30° and particularly preferably of at least 40°.

Advantageously, only a very short or no tubular feed line section is at the same time upstream of the cone towards the turbocharger, but rather the cone is optionally directly connected to the turbocharger. Should a tubular feed line section be provided, however, for example in order to make available a sufficiently large return flow area for the exhaust gas with a cup-shaped component, this section should not exceed a length of 20 mm (millimeters), in particular it should not be longer than 10 mm or even only 8 mm. With such a construction, notably the exhaust gas stream generated by the turbocharger is used for an effective flow towards the honeycomb body. The turbocharger generates a type of swirl-stream, which is advantageously maintained and thus results in an intensive contact of the uniformly mixed exhaust gas stream.

In accordance with an additional feature of the invention, the housing and the at least one catalytic converter are constructed concentrically, preferably coaxially. The concentric or coaxial structure of the catalytic converter and housing advantageously allows the exhaust gas after-treatment unit to be constructed in a particularly simple manner, in particular catalytic converters in cylindrical construction which are conventional per se can thus be used. The coaxial structure advantageously offers only low pressure losses in the return flow region, with a simultaneous simple structure of the exhaust gas after-treatment unit. Furthermore, the concentric or coaxial structure of the catalytic converter and housing simplifies the construction of the flow deflector. If the housing and catalytic converter have substantially a cylinder geometry and if the exhaust gas flows axially through the catalytic converter, the flow deflector can be constructed in a particularly simple manner by formation of a torus with the smallest possible internal radius, in the ideal case of zero. If the exhaust gas flows through the catalytic converter substantially radially, the housing itself forms the flow deflector, which ensures deflection of the exhaust gas from the radial flow direction into the return flow direction.

In accordance with yet another feature of the invention, the at least one return flow region is constructed outside the at least one catalytic converter. The construction of the return flow region outside the at least one catalytic converter advantageously ensures rapid starting up properties of the catalytic converter, uniform heating up of the catalytic converter with prevention of the formation of thermal gradients and a simple structural layout both of the catalytic converter and of the housing, since a conventional catalytic converter with a honeycomb structure of ceramic or metal, optionally an extruded honeycomb structure, can be employed inside the housing. In an advantageous manner, it is possible to fix the catalytic converter with holding devices, for example thin bars, which point radially from the catalytic converter outwards in the direction of the housing, without the pressure loss in the return flow region being considerably increased. Other holding devices are also possible according to the invention, in particular it is also advantageous to fix the catalytic converter only through the use of the gas feed line.

In accordance with yet a further feature of the invention, the hollow spaces of the at least one catalytic converter each have a first through-flow cross-section, and an inner region E-81181 with a second through-flow cross-section is constructed as the return flow region within the catalytic converter. In this case the second through-flow cross-section is significantly greater than the first through-flow cross-section. This allows, for example, the use of catalytic converters in the form of hollow cylinders, the flow-through cross-section of which is an annulus with hollow spaces of a first through-flow cross-section.

In accordance with yet an added feature of the invention, the second flow-through cross-section of the return flow region is substantially the same size as the sum of the first flow-through cross-sections of the catalytic converter. This advantageously prevents a pressure loss during the deflection of the flow. However, it is equally as advantageous to construct the second through-flow cross-section to be greater than the sum of the first flow-through cross-sections, in order to thus slow down the flow in the return flow region and to increase the heat transfer to the catalytic converter in the cold starting phase.

In accordance with yet an additional feature of the invention, the housing has a first length L1, the catalytic converter has a second length L2, and the first length of the housing and the second length of the catalytic converter are substantially identical. The construction of the catalytic converter in an identical length to the length of the housing allows holding of the catalytic converter in the housing in a simple manner and a simple construction both of the flow deflector and of the gas removal and feed line.

In accordance with again another feature of the invention, the housing has a diameter D, the quotient of the first length L1 and the diameter D of the housing is greater than or equal to 0.3 and less than or equal to 1.5, preferably greater than or equal to 0.3 and less than or equal to 1, and particularly preferably about 0.5. That is to say, the following equation applies to the first length L1 and the diameter D of the housing: 0.3≦L1/D≦1.5

In accordance with again a further feature of the invention, the return flow region has a pressure loss which is less than or equal to the pressure loss of the inflow region, in particular less than or equal to the pressure loss of a pipe of the first length and a diameter which corresponds to the diameter of the feed line.

In accordance with again an added feature of the invention, the at least one gas feed line has a first longitudinal axis, the at least one gas removal line has a second longitudinal axis, and a projection of the first and the second longitudinal axis onto a plane which includes the first end surface of the catalytic converter encloses an angle which is greater than 60° (degrees). Such an angle constellation between the gas removal line and the gas feed line advantageously allows the utilization of even the smallest free hollow spaces upon installation close to the engine, for example of very narrow pocket holes.

In accordance with again an additional feature of the invention, the gas feed line and the first end surface of the at least one catalytic converter are connected to one another in the form of a push-fit. The construction of the connection between the gas feed line and the first end surface in the form of a push-fit advantageously allows the construction of a substantially gas-tight connection, and at the same time allows a different thermal expansion, which in the case of a simple weld connection can easily lead to severing of the connection. A substantially gas-tight connection between the gas feed line and the first end surface of the at least one catalytic converter can thus be ensured in an advantageous manner even in the case of different thermal expansion properties.

In accordance with a concomitant feature of the invention, the catalytic converter is constructed of ceramic. Construction of the catalytic converter as an extruded component is also advantageous. According to a further advantageous construction, the catalytic converter can also be constructed from at least one metallic layer. In this connection, it is particularly advantageous that the catalytic converter is constructed:

-   -   a) by winding up at least one at least partly structured         metallic layer or at least one substantially smooth and at least         one at least partly structured metallic layer, or     -   b) by stacking a plurality of substantially smooth and at least         partly structured metallic layers and subsequently winding a         plurality of stacks.

This allows the construction both of spiral honeycomb bodies and of metallic honeycomb bodies with stacks intertwined in S-shaped or involuted form. In particular, in the case of metallic catalytic converters it is advantageous according to the invention for them to be provided with structures constructed transversally to the extension of the hollow space or longitudinally to the extension of the hollow space, to provide holes in the metallic layers, as well as to construct at least a part of the metallic layers of material which is at least partly permeable to a fluid.

With the objects of the invention in view, there is also provided a process for exhaust gas after-treatment, in particular close to an internal combustion engine of an automobile. The process comprises the following steps:

a) An exhaust gas flow is guided through an inflow region of an exhaust gas after-treatment unit according to the invention in an inflow direction and at least parts of the exhaust gas are catalytically converted in the inflow region.

b) A direction of flow of the exhaust gas is deflected from the inflow direction into a return flow direction.

c) The exhaust gas flow is guided through a substantially free flow-through return flow region in a return flow direction.

The advantages and details described above for the exhaust gas after-treatment unit according to the invention can be applied in the same manner to the process according to the invention for exhaust gas after-treatment.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is illustrated and described herein as embodied in an exhaust gas after-treatment unit with a countercurrent housing and a corresponding process for exhaust gas after-treatment, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic, longitudinal-sectional view of an exhaust gas after-treatment unit according to the invention;

FIG. 2 is a cross-sectional view of a honeycomb body;

FIG. 3 is a perspective view of a housing with a honeycomb body installed;

FIG. 4 is a perspective view of a second embodiment of an exhaust gas after-treatment unit according to the invention;

FIG. 5 is a cross-sectional view of a section through the second embodiment of the exhaust gas after-treatment unit; and

FIG. 6 is an enlarged, longitudinal-sectional view of a third embodiment of an exhaust gas after-treatment unit according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawings in detail and first, particularly, to FIG. 1 thereof, there is seen a longitudinal section through a first embodiment of an exhaust gas after-treatment unit 1 according to the invention. The exhaust gas after-treatment unit 1 has a housing 2 with a honeycomb body 3, which serves as a catalytic converter. The honeycomb body 3 is surrounded by a casing tube 4 and is fixed in the housing 2 with holding devices 5. These holding devices 5 are predominantly constructed as bars which do not substantially reduce the size of a free through-flow cross-section of a return flow region 6. The phrase free through-flow cross-section, means in particular that no honeycomb structure is placed in the return flow region. The honeycomb body 3 can be constructed both as a ceramic and as a metallic honeycomb body 3. An example of a metallic honeycomb body can be seen in FIG. 2. A feed line 13 is provided with a cone 35, which conducts exhaust gas directly to a first end surface 14 of the honeycomb body 3. This cone 35 has an opening angle 33 of at least 20°. A very short tubular feed line section which is disposed upstream of the cone 35 towards a non-illustrated turbocharger, does not exceed a length 34 of 20 mm (millimeters).

FIG. 2 shows the honeycomb body 3 which has the casing tube 4. A honeycomb structure 7 is fixed in the casing tube 4. The structure 7 is formed of metallic layers 8, 9. In order to form the honeycomb structure 7, substantially smooth metallic layers 8 and at least partly structured metallic layers 9 are stacked alternately and several stacks are joined to one another in the same direction. For simplicity, the at least partly structured metallic layers 9 are shown only in a partial region. The substantially smooth metallic layers 8 and the at least partly structured metallic layers 9 form hollow spaces or channels 10.

Sheet metal layers having a thickness of less than 80 μm, preferably less than 40 μm, particularly preferably less than 25 μm, can be used as the metallic layers. It is equally possible for the substantially smooth metallic layers 8 and/or the at least partly structured metallic layers 9 to be constructed at least partly from a material through which a fluid can at least partially flow, for example a metallic sintered non-woven. It is furthermore possible, according to the invention, to introduce holes and/or structures of any type into the substantially smooth metallic layers 8 and/or the at least partly structured metallic layers 9. In particular, it is also possible to close some of the channels 10. The introduction of holes having dimensions greater than the structurally recurring length of the at least partly structured metallic layers 9 is also possible according to the invention.

FIG. 1 shows that the housing 2 of the exhaust gas after-treatment unit 1 according to the invention has two flow regions. The channels 10 of the honeycomb body 3 form an inflow region 11, while a region of the housing between the casing tube 4 and walls of the housing 2 forms the return flow region 6. The honeycomb body 3 serves as a catalytic converter, i.e. it is as a rule provided with a catalytically active coating, for example a washcoat, which includes, for example, noble metal catalyst particles, such as platinum or rhodium. In the present embodiment, the exhaust gas flows axially through the honeycomb body 3. An exhaust gas stream flowing through the honeycomb body 3 is at least partly catalytically converted in the honeycomb body 3. In contrast, an exhaust gas stream flowing through the return flow region 6 is not catalytically converted.

Each of the channels 10 has a first through-flow cross-section, while the return flow region 6 has a second free through-flow cross-section. The phrase “free flow-through” means that the second flow-through cross-section of the return flow region 6 is significantly greater than the first flow-through cross-section of a channel 10.

During operation of the exhaust gas after-treatment unit 1, an exhaust gas stream 12 is introduced through the gas feed line 13 into the exhaust gas after-treatment unit 1. The gas feed line 13 is connected in a substantially gas-tight manner to the casing tube 4 of the honeycomb body 3 in the region of the first end surface 14 of the honeycomb body, so that there is a substantially gas-tight connection between the gas feed line 13 and the inflow region 11. The exhaust stream 12 thus enters the honeycomb body 3 substantially completely. The exhaust gas stream 12 flows through this honeycomb body 3 in an inflow direction 15. During this flow, an at least partial conversion of at least parts of the exhaust gas stream 12 takes place. The exhaust gas stream 12 leaves the honeycomb body 3 through a second end surface 16. A flow deflector 17 is adjacent the region of the second end surface 16 in the inflow direction 15. The flow deflector 17 is connected to the housing 2 in a substantially gas-tight manner. The flow deflector 17 has a depression 18 and a torus-shaped elevation 19. The highest elevations each lie in the axial direction of the honeycomb body 3 opposite the center of the return flow region 6, while the depression 18 lies opposite the center of the cylindrical honeycomb body 3 in the axial direction. Other constructions of the flow deflector 17 are also possible according to the invention. The flow deflector 17 leads to a deflection 20 of the exhaust gas stream 12 from the inflow direction 15 into a return flow direction 21. In the present case, this is even an inversion of the exhaust gas stream, i.e. a deflection by substantially 180°. During this inversion, the exhaust gas stream 12 is deflected from the inflow region 11 into the return flow region 6. The flow deflector 17 can optionally have a thermal insulation 22.

A collector 23 is furthermore connected to the housing 2 with a connection that is substantially gas-tight in construction. The collector 23 includes a cup-shaped component 24 and a gas removal line 25. An at least partly converted gas stream 26 leaves the exhaust gas after-treatment unit 1 through the gas removal line 25. The gas removal line 25 and/or the gas feed line 13 have a connection for a turbocharger.

As mentioned above, during operation, the exhaust gas stream 12 flows through the gas feed line 13 into the honeycomb body 3. An at least partial catalytic conversion of at least a part of the exhaust gas stream 12 takes place in this manner. After flowing through the honeycomb body 3 in the inflow direction 15, the deflection 20 in the direction of flow takes place in the flow deflector 17. The exhaust gas stream 12 then flows in the return flow direction 21 through the return flow region 6. No catalytic conversion takes place in the return flow region 6, which is substantially an undivided flow chamber. The gas stream flowing through the return flow region 6 as a rule is heated as compared with the exhaust gas stream 12 flowing in, since the catalytic conversion in the honeycomb body 3 as a rule takes place exothermically. The gas stream flowing through the return flow region 6 is thus advantageously used for heating the honeycomb body 3. In the cold start phase as well, in which no heating of the gas stream in the honeycomb body 3 takes place because the exothermic reaction has not yet started up, the recycling of the exhaust gas stream can advantageously be used for heating up the honeycomb body 3, since upon cold start of an internal combustion engine elevated temperatures are rapidly achieved which, although below the starting up temperature of the catalytic conversion in the catalytic converter 3, are above the ambient temperature of the environment of the honeycomb body 3. This leads to significantly shorter starting up times of the catalytic reaction in the honeycomb body 3. The optional thermal insulation 22 of the flow deflector 17 also prevents heat losses and therefore improves the starting up properties of the honeycomb body 3. Furthermore, the return flow of the hot exhaust gas means that lower thermal gradients build up over the honeycomb body as compared with conventional exhaust gas after-treatment units. This results in an improved service life of the honeycomb body.

A push-fit or sliding-fit can advantageously be used for the connection between the gas feed line 13 and the casing tube 4. This renders possible a gas-tight connection even in the case of different thermal expansions of the two components.

Through the use of the return flow principle, in particular due to the fact that the gas feed line 13 and the gas removal line 25 are both constructed in the region of the first end surface 14 of the honeycomb body 3, the utilization of even small free spaces in the region of the engine compartment of an automobile, for example pocket holes, is possible. The exhaust gas after-treatment unit 1 can thus be installed as close to the engine as possible. In this way, higher temperatures are reached more quickly in the exhaust gas, so that in this way the starting up properties of the honeycomb body 3 are also improved. The gas feed line 13 has a first longitudinal axis 27. The gas removal line 25 has a second longitudinal axis 28. In order to render possible an installation of the exhaust gas after-treatment unit 1 which is as space-saving as possible, it is advantageous if the angle of the projections of the first longitudinal axis 27 and the second longitudinal axis 28 onto a plane which includes the first end surface 14 is greater than 60 degrees.

In a honeycomb body 3 according to the invention, the return flow region 6 has a pressure loss which is less than or equal to the pressure loss in the inflow region 11. It is preferable in this case for the pressure loss in the return flow region 6 to be less than or equal to a pressure loss encountered by a pipe having a first length L1 and a diameter which corresponds to the diameter 32 of a feed line 31.

FIG. 3 shows a housing 2 according to the invention with the honeycomb body 3 inserted. The honeycomb body 3 is constructed to be coaxial with the housing 2. The casing tube 4 of the honeycomb body 3 is connected to the housing 2 by the holding devices 5. The channels 10 of the honeycomb body 3, which are not drawn in for clarity, form the inflow region 11, while the region of the housing between the housing wall and the casing tube 4 forms the return flow region 6. The housing 2 has a first length L1 and a diameter D. The honeycomb body 3 has a second length L2. In the present embodiment, the first length L1 is identical to the second length L2. A so-called pancake shape is preferred for the exhaust gas after-treatment unit according to the invention, i.e. for the ratio of L1/D, the equation 0.3≦L1/D≦1 preferably applies. It is particularly preferable in this case for the ratio of L1/D to be about 0.5. However, other ratios of L1/D are also possible according to the invention.

FIG. 4 shows a diagrammatic illustration of a second embodiment of an exhaust gas after-treatment unit 1 according to the invention. In this case, four non-illustrated honeycomb bodies 3 which are charged with exhaust gas through four gas feed lines 13 are fixed in the housing 2 of the exhaust gas after-treatment unit 1. A gas removal line 25 is furthermore provided, so that this embodiment of an exhaust gas after-treatment unit according to the invention can be employed as a collector. It is likewise possible according to the invention to provide two or more gas removal lines 25 instead of one gas removal line 25, in order to thus be able to realize, for example, multi-lane or multi-tract exhaust gas units. The cup-shaped components 24 are accordingly connected to one another. The flow deflector 17 is constructed in such a way that an effective non-illustrated deflection 20 from the inflow regions 11 into the particular return flow regions 6 also takes place in this embodiment. In this case as well, the flow deflector 17 is constructed with depressions 18 and elevations 19. The depressions 18 in each case are constructed centrally with respect to the honeycomb body 3.

FIG. 5 shows a diagrammatic view of a section through the embodiment shown in FIG. 4, which is taken along a line V-V. This cross-section shows the housing 2 with the four honeycomb bodies 3 fixed thereto. In this cross-section, the casing tubes 4 form the boundary between the inflow regions 11 and the return flow region 6.

FIG. 6 shows a third embodiment of an exhaust gas after-treatment unit 1 according to the invention, which has a radial flow-through honeycomb body 3. As is known from the prior art, this honeycomb body 3 is constructed from discs 29 with non-illustrated macrostructures which form channels 10 that lead in an arc shape from a central flow region 30 to the return flow region 6. The exhaust gas stream 12 to be converted flows axially through the gas feed line 13 and through the first end surface 14 into the central flow region 30. As a result of the second end surface 16 of the honeycomb body 3 being closed, the gas stream is deflected into the radial flow channels 10, as is indicated by arrows. The inflow direction 15 of the inflow region 11 formed by the channels 10 is thus directed radially from the inside outwards. The housing 2 serves as the flow deflector 17 and, after exiting of the gas from the channels 10, effects a deflection 20 of the gas stream in the return flow direction 21 in the return flow region 6. In contrast to an axial flow-through honeycomb body in which a deflection of, for example, substantially 180° takes place, in a radial flow-through honeycomb body 3 the gas stream is deflected by about 90°.

The exhaust gas flows from the return flow region 6 into the cup-shaped component 24. From there, the converted gas stream 26 leaves the exhaust gas after-treatment unit 1 through the gas removal line 25. In this embodiment also, the gas feed line 13 and the gas removal line 25 are in the region of the first end surface 14 of the honeycomb body.

With an exhaust gas after-treatment unit 1 according to the invention, at least partial catalytic conversion of exhaust gases can advantageously take place even in the event of very limited free space for accommodating an exhaust gas after-treatment unit 1. This is possible on the basis of the countercurrent principle in the housing 2. An exhaust gas after-treatment unit 1 according to the invention is furthermore distinguished by improved starting up properties and lower thermal alternating stresses as compared with conventional exhaust gas after-treatment units. 

1. An exhaust gas after-treatment unit, comprising: a housing; at least one catalytic converter disposed in said housing and defining at least one substantially free flow-through return flow region disposed within said housing and surrounding said at least one catalytic converter, said at least one catalytic converter having a first end surface, a second end surface and hollow spaces through which a fluid can flow in an inflow direction; at least one gas feed line connected to said first end surface of said at least one catalytic converter; at least one gas removal line substantially gas-tightly connected to said at least one return flow region; and at least one flow deflector effecting a deflection of the fluid from said at least one catalytic converter into said substantially free flow-through return flow region of said housing.
 2. The exhaust gas after-treatment unit according to claim 1, wherein said at least one gas feed line and said at least one gas removal line are disposed in vicinity of said first end surface of said at least one catalytic converter.
 3. The exhaust gas after-treatment unit according to claim 1, wherein said housing is a manifold.
 4. The exhaust gas after-treatment unit according to claim 1, wherein said housing is a collector.
 5. The exhaust gas after-treatment unit according to claim 1, wherein at least one of said at least one gas removal line or said at least one gas feed line has a connection for a turbocharger.
 6. The exhaust gas after-treatment unit according to claim 1, wherein said housing and said at least one catalytic converter are concentric.
 7. The exhaust gas after-treatment unit according to claim 1, wherein said housing and said at least one catalytic converter are coaxial.
 8. The exhaust gas after-treatment unit according to claim 1, wherein said at least one return flow region is disposed outside said at least one catalytic converter.
 9. The exhaust gas after-treatment unit according to claim 1, wherein said hollow spaces of said at least one catalytic converter each have a first through-flow cross-section, said return flow region is an inner region within said at least one catalytic converter having a second through-flow cross-section, and said second cross-section is significantly greater than said first cross-section.
 10. The exhaust gas after-treatment unit according to claim 9, wherein said second flow-through cross-section of said return flow region is substantially the same size as a sum of said first flow-through cross-sections of said at least one catalytic converter.
 11. The exhaust gas after-treatment unit according to claim 1, wherein said housing has a first length, said at least one catalytic converter has a second length, and said first length of said housing and said second length of said at least one catalytic converter are substantially identical.
 12. The exhaust gas after-treatment unit according to claim 11, wherein said housing has a diameter, and a quotient of said first length and said diameter of said housing is at least 0.3 and at most 1.5.
 13. The exhaust gas after-treatment unit according to claim 11, wherein said housing has a diameter, and a quotient of said first length and said diameter of said housing is at least 0.3 and at most
 1. 14. The exhaust gas after-treatment unit according to claim 11, wherein said housing has a diameter, and a quotient of said first length and said diameter of said housing is approximately 0.5.
 15. The exhaust gas after-treatment unit according to claim 1, wherein said hollow spaces form an inflow region, and said return flow region has a pressure loss being at most equal to a pressure loss of said inflow region.
 16. The exhaust gas after-treatment unit according to claim 11, wherein said return flow region has a pressure loss being at most equal to a pressure loss of a pipe having said first length and a diameter corresponding to a diameter of said at least one feed line.
 17. The exhaust gas after-treatment unit according to claim 1, wherein said at least one gas feed line has a first longitudinal axis, said at least one gas removal line has a second longitudinal axis, and a projection of said first and second longitudinal axes onto a plane including said first end surface of said at least one catalytic converter encloses an angle greater than 60°.
 18. The exhaust gas after-treatment unit according to claim 1, wherein said at least one gas feed line and said first end surface of said at least one catalytic converter are connected to one another with a push-fit.
 19. The exhaust gas after-treatment unit according to claim 1, wherein said at least one catalytic converter is formed of ceramic.
 20. The exhaust gas after-treatment unit according to claim 1, wherein said at least one catalytic converter is extruded.
 21. The exhaust gas after-treatment unit according to claim 1, wherein said at least one catalytic converter is formed of at least one metallic layer.
 22. The exhaust gas after-treatment unit according to claim 21, wherein said at least one catalytic converter is constructed by winding up at least one at least partly structured metallic layer.
 23. The exhaust gas after-treatment unit according to claim 21, wherein said at least one catalytic converter is constructed by winding up at least one substantially smooth and at least one at least partly structured metallic layer.
 24. The exhaust gas after-treatment unit according to claim 21, wherein said at least one catalytic converter is constructed by stacking a plurality of substantially smooth and at least partly structured metallic layers and subsequently winding a plurality of stacks.
 25. The exhaust gas after-treatment unit according to claim 1, wherein said housing is disposed close to an internal combustion engine of an automobile.
 26. A process for exhaust gas after-treatment, which comprises the following steps: a) providing an exhaust gas after-treatment unit; b) guiding an exhaust gas flow through an inflow region of the exhaust gas after-treatment unit in an inflow direction and catalytically converting at least parts of the exhaust gas in the inflow region; c) deflecting a direction of flow of the exhaust gas from the inflow direction into a return flow direction; and d) guiding the exhaust gas flow through a substantially free flow-through return flow region in a return flow direction.
 27. A process for exhaust gas after-treatment, which comprises the following steps: a) guiding an exhaust gas flow through an inflow region of an exhaust gas after-treatment unit according to claim 1 in an inflow direction and catalytically converting at least parts of the exhaust gas in the inflow region; b) deflecting a direction of flow of the exhaust gas from the inflow direction into a return flow direction; and c) guiding the exhaust gas flow through a substantially free flow-through return flow region in a return flow direction.
 28. The process according to claim 26, which further comprises supplying the exhaust gas from an internal combustion engine of an automobile.
 29. The process according to claim 27, which further comprises supplying the exhaust gas from an internal combustion engine of an automobile. 